Go back to the first one and try the bonus levels, then we'll talk. On the 10th level of Frontiers now, you aren't playing veteran are you? That's not meant for prior to having all the upgrades.
Why would you need to buy gems though? I have 2000+ already, and the freeze thingies are only 250. Definitely not finding it overly hard, but I did lose at the last wave of the 9th level, and upon trying again with a more efficient strategy finished it perfect. Just watch a youtube video if you're stuck if you can find one, because the whole game is beatable without any special heroes or gem-related powers.
First one is the same way though, you get like 3 heroes in it, and all the rest are IAP. This one is structured in a way that encourages the IAP of the heroes less than the first game, because you actually need to use your heroes to level them... and going by my progress, you can't even max level the starting 3 heroes prior to finishing the campaign. So any extra heroes you buy also need to be leveled, so buying a bunch is counterproductive, since you'd have to keep playing the same levels over and over to level them... where in the first game, there was no penalty to having all 9 or 10 of them.
